1. Field of the Invention
The present invention relates to a flat panel display and, more specifically, to a pixel circuit in an organic light emitting device capable of realizing high gradation by self-compensating a threshold voltage of a transistor that drives an electroluminescent (EL) element, and a method for driving the same.
2. Description of the Related Art
Normally, an organic light emitting device may be classified into a passive matrix organic light emitting diode (OLED) and an active matrix OLED (AMOLED), and can be classified into a current driving OLED and a voltage driving OLED depending on the manner in which the EL element is driven.
A typical AMOLED is generally composed of a plurality of gate lines, a plurality of data lines, a plurality of power lines, and a plurality of pixels connected to the lines and arranged in a matrix form. Each pixel is normally composed of: an EL element; two transistors, in which one is a switching transistor for transferring a data signal while the other is a driving transistor for driving the EL element depending on the data signal; and one capacitor for maintaining the data voltage.
Although this AMOLED has an advantage in that power consumption is low, current intensity flowing through the EL element changeing over time, causing display nonuniformity, can be a problem. This results from a change in voltage between the gate and the source of the driving transistor for driving the EL element, namely, the threshold voltage of the driving transistor, which leads to a change in the current flowing through the EL element. Since the threshold voltage of a thin film transistor for the driving transistor changes depending on manufacturing process parameters, it becomes difficult to manufacture transistors in the AMOLED so that all of the transistors have the same threshold voltage. Thus, there are threshold voltage deviations between pixels.
In order to solve this voltage deivation problem, a method has been developed for compensating the threshold voltage depending on manufacturing process parameters by adding a transistor for threshold voltage compensation. U.S. Pat. No. 6,229,506 ('506 patent) discloses an organic light emitting device for compensating the threshold voltage deviation. The '506 patent discloses a pixel structure in which a current source adjusts a voltage between the source and the gate of a driving transistor with respect to an overdrive voltage thereof and compensates the threshold voltage deviation of the driving transistor. The organic light emitting device in the '506 patent performs a two-step operation involving a data load (data write) step and a continuous light-emitting step, in which a current source adjusts a voltage between the source and the gate of the driving transistor with respect to the overdrive voltage and compensates the threshold voltage deviation of the driving transistor.
However, the organic light emitting device as described above employs a current driving approach for driving the EL element which depends on a data signal current level applied from the current source and has difficulty in charging a data line. Because a parasitic capacitance of the data line is relatively larger while the current level of the data signal provided from the current source is relatively smaller, the data becomes unstable as well as considerably long time is required to charge the data line.
In order to solve the data line charging problem in the current driving approach, an organic light emitting device having a mirror type pixel structure has been proposed. FIG. 1 shows a pixel circuit of a voltage driving manner having a mirror type in a conventional voltage driving organic light emitting device.
Referring to FIG. 1, the pixel circuit comprises first P-type transistor T11 in which the gate of the first transistor is connected to current scan signal SCAN[n] applied to an associated scan line of a plurality of gate lines. Data signal VDATAm applied to an associated data line of a plurality of data lines is applied to its source. Second P-type transistor T12 in which a previous scan signal SCAN[n-1] is applied to a scan line just before the current scan line is applied to its gate. Initialization voltage Vinti is applied to its drain. Third and fourth P-type transistors T13 and T14 have a mirror type configuration. Fifth N-type transistor T15 in which previous scan signal SCAN[n-1] is applied to its gate has its drain coupled to the drain of fourth transistor T14. EL element EL11 is connected between fifth transistor T15 and ground voltage VSS. First capacitor C11 is connected between the gate and the source of fourth transistor T14.
Operation of the pixel in the organic light emitting device having the above-described structure will be described with reference to an operation waveform diagram of FIG. 2. Here, it is assumed that a scan line to be currently driven is the n-th scan line. A scan signal applied to the n-th scan line is SCAN[n]. A scan line driven before the current scan line is the (n-1)th scan line. A scan signal applied to the (n-i)th scan line is SCAN[n-1].
First of all, in initializing the operation, if predetermined levels of previous scan signal SCAN[n-1] and current scan signal SCAN[n] are applied thereto, that is, if a low level of previous scan signal SCAN[n-1] and a high level of current scan signal SCAN[n] are applied thereto, transistor T12 is turned on and transistors T11 and T15 are turned off, such that mirror-type transistors T13 and T14 are also turned off. Accordingly, the data stored in capacitor C11 is initialized through transistor T12 to initialization voltage Vinti.
Meanwhile, in programming data, if predetermined levels of previous scan signal SCAN[n-1] and current scan signal are applied thereto, that is, if a high level of previous scan signal SCAN[n-1] and a low level of current scan signal SCAN[n] are applied thereto, transistor T12 is turned off and transistor T11 is turned on, such that mirror-type transistors T13 and T14 are turned on.
Thus, a data signal voltage level VDATAm applied to the data line is transferred through transistor T13 to the gate of driving transistor T14. At this time, since transistor T15 is turned on by previous scan signal SCAN[n-1], a driving current corresponding to the data signal voltage VDATAm applied to the gate of driving transistor T14 flows into EL element EL 11 for its light-emitting.
The voltage applied to the gate of transistor T14 becomes VDATA-VTH(T13), and the current flowing through EL element EL11 is represented by the following Expression 1.
                                                                        I                EL11                            =                            ⁢                                                β                  2                                ⁢                                                      (                                                                  V                                                  GS                          ⁡                                                      (                            T14                            )                                                                                              -                                              V                                                  TH                          ⁡                                                      (                            T14                            )                                                                                                                )                                    2                                                                                                        =                            ⁢                                                β                  2                                ⁢                                                      (                                                                  V                        DD                                            -                                              V                        DATA                                            +                                              V                                                  TH                          ⁡                                                      (                            T13                            )                                                                                              -                                              V                                                  TH                          ⁡                                                      (                            T14                            )                                                                                                                )                                    2                                                                                        (        1        )            
Where, IEL 11 represents the current flowing through organic EL element EL 11, VGS(T14) represents a voltage between the source and the gate of transistor T14, VTH(13) represents a threshold voltage of transistor T13, VDATA represents a data voltage, and β represents a constant value, respectively.
At this time, if threshold voltages of transistors T13 and T14 for the current mirror are identical with each other, i.e., if VTH(T13)=VTH(T14), the threshold voltage of the transistor can be compensated, thereby maintaining the driving current of EL element EL 11 to be uniform.
However, although transistors T13 and T14 configuring the current mirror are arranged adjacent to each other on a substrate in the voltage driving manner of the current mirror type as described above, it is very difficult to obtain the same threshold voltage due to the manufacturing process parameters of TFT. Therefore, there is a problem that it is difficult to obtain a uniform driving current due to deviation of the threshold voltage of TFT, resulting in degraded image quality.
A technique for solving the image quality degradation due to the threshold voltage deviation between TFTs for the current mirror in the voltage driving manner of the current mirror type as described above is disclosed in U.S. Pat. No. 6,362,798 ('798 patent). In the '798 patent, a compensating thin film transistor having a diode form is connected to a gate of the driving transistor in order to compensate the threshold voltage of the driving transistor. However, there is a problem with the '798 patent that when threshold voltages of the thin film transistor for compensation and the thin film transistor for driving EL element drive are different from each other, threshold voltage deviation of the driving transistor is not compensated, as well.